One way of using arrays of concentric annular transducers in ultrasonic imaging apparatus is to excite the transducers with pulses in such rapid sequence that the array produces a beam of pressure waves that is focussed at a given point along the axis of the array. The beam is made to sweep a sector by rocking the array or to sweep a rectangular area by moving the array along a line. In either case, the electrical signals produced by the different transducers in response to reflections of the pressure waves are subjected to respectively different delays so as to bring them into focus before they are processed.
If the body structure being examined were stationary, improved results could be attained by focussing each transmitted pulse at successive pixels along a line and creating a composite image from the reflections thereof; but where the apparatus is being used to produce an image of moving structures such as a fetus, the interval that must exist between successive transmitted pulses in order to permit echoes to return from maximum range is so great that nothing approaching a real-time image can be obtained.
Even if the transmitted pulses could be focussed in this manner, safety requires that the amount of power, or the "hot spot", at the focal point be kept under prescribed limits. This means that a proper combination of the amplitude and frequency of the transmitted pressure waves must be used.
Furthermore, when the transducers are excited at substantially the same time, it is impossible to provide dynamic apodization in both transmit and receive so that a difference in the texture or speckle of an image due to the presence of a tumor or other abnormality can be more easily observed.